Conventionally known methods for increasing the viscosity of an external composition include a method for incorporating into the composition a viscosity control agent or a thickening agent; for example, a polysaccharide such as xanthan gum, a hydrophilic synthetic polymer such as a polyacrylic acid, or a clay mineral such as bentonite.
When a polysaccharide such as xanthan gum is incorporated, as a viscosity control agent, into an external composition, the composition involves problems in terms of sensation during use; for example, the composition provides a sticky sensation during use, although such a polysaccharide exhibits excellent stability in the composition into which a pharmaceutical ingredient or a salt is incorporated. When a hydrophilic synthetic polymer such as polyacrylic acid is incorporated into an external composition, the composition provides good sensation during use; i.e., the composition provides no sticky sensation during use, but provides a refreshing sensation during use. However, such a hydrophilic synthetic polymer has low resistance to a salt or an ionic substance. Therefore, when a large amount of a salt or a pharmaceutical ingredient such as a whitening ingredient; for example, L-ascorbic acid (i.e., vitamin C) or arbutin, is incorporated into the composition, the composition involves problems including lowering of the viscosity of the composition. When a clay mineral such as bentonite is incorporated, as a viscosity control agent, into an external composition, the composition involves problems in terms of sensation during use; for example, the composition provides a frictional sensation during use.
An object of the present invention is to provide a new type of viscosity control agent which, when incorporated as an ingredient into an external composition, imparts the composition with excellent sensation during use; i.e., free of sticky sensation or frictional sensation. Furthermore, even when a large amount of a salt or a pharmaceutical ingredient such as a whitening ingredient is incorporated into the composition, the viscosity of the composition is not lowered, the composition exhibits long-term stability, and separation of water does not occur. Another object of the present invention is to provide an external composition comprising the viscosity control agent.